US President Barack Obama warned that thousands of foreign militants are still flooding into Syria to fight, as he urged greater efforts to halt the flow, particularly through the porous Turkish border.

The United States is leading a coalition aimed at eliminating Islamic State militants in Iraq that has unleashed 4,000 air strikes on IS positions, helping Baghdad to drive the militants out of some of their strongholds.

The militants launched a lightning offensive a year ago and quickly snatched over a third of Iraq's territory, declaring a "caliphate" with parts of Syria they also control.

They still hold much of western Iraq and have in recent weeks proven capable of snatching new territory.